A concrete mixer truck is a vehicle that transports fresh concrete such as mortar or ready-mixed concrete from a fresh concrete factory to a construction site in a mixer drum that is carried on a frame to be free to rotate.
To prevent the fresh concrete from decreasing in quality and hardening, the mixer drum is rotated normally as the concrete mixer truck transports the fresh concrete such that the fresh concrete is constantly agitated by a plurality of spiral blades disposed in the mixer drum. Further, the concrete mixer truck is configured such that the fresh concrete in the mixer drum can be discharged by rotating the mixer drum in an opposite direction to the normal rotation. When the concrete mixer truck arrives at a concrete pouring site, the mixer drum is rotated in reverse so that the fresh concrete is supplied to a pouring location.
In this type of concrete mixer truck, the mixer drum must be rotated constantly until the fresh concrete is discharged. An engine of the concrete mixer truck is typically used as a drive source of the mixer drum. More specifically, a rotary driving force of the engine is transmitted to a hydraulic pump via a PTO (Power Take Off), whereupon working oil discharged from the hydraulic pump is supplied to a hydraulic motor. The hydraulic motor is driven to rotate by the working oil, and the mixer drum is driven to rotate by the rotation of the hydraulic motor.
In a mixer drum driving apparatus that drives a mixer drum using only an engine, an engine rotation speed must be increased when the mixer drum is to be rotated at high speed or the like. When the engine rotation speed is increased in this manner, noise is generated and fuel consumption rises.
Further, the mixer drum must be rotated constantly for as long as the fresh concrete is carried in the mixer drum to prevent the fresh concrete from hardening and so on, meaning that the engine cannot be stopped. It is therefore necessary to continue to drive the engine even when the concrete mixer truck is stopped at the pouring site while waiting its turn to discharge the fresh concrete.
JP2007-278430A discloses a mixer drum driving apparatus that drives a mixer drum to rotate by driving an auxiliary hydraulic pump using a rotary motor in addition to a main hydraulic pump driven using an engine. In this mixer drum driving apparatus, the main pump driven by the engine is assisted by the auxiliary pump driven by the rotary motor, and therefore noise generation can be suppressed and fuel consumption can be reduced.